Blunt was born in Burnemouth, the son of an Anglican minister. He was the younger brother of the artist and writer Wilfred Blunt, who was also gay. He was educated at Marlborough, where he displayed a precocious brilliance and developed the passion for art history that was to dominate his professional life. In 1926, he went to Trinity College, Cambridge, where he met Guy Burgess and they were probably lovers, though Blunt later denied this.

As a Cambridge don, he recruited for the Soviet secret service, and, as a member of the British Secret Service (1940 - 1945), passed information to the Russians. In 1951 he assisted the defection of Guy Burgess and Donald Maclean. Unmasked in 1964, he was given immunity after his confession, but was stripped of his knighthood in 1979 when the affair became public.